The present invention relates generally to rotating electrical machines, and more particularly, to a stator structure configured to generate a reduced amount of audible noise.
Alternators are rotating electrical machines used in vehicles for generating electric power to charge the battery when the rotator of the alternator is turning at a sufficient speed. Those skilled in the art will recognize that alternators are also referred to as generators. In the design of automotive vehicles, manufacturers are continually trying to reduce the audible noise generated from various components in the vehicle. Alternators generate audible noise due to the magnetic force in the alternator and other factors, such as fan noise.
Referring now to FIG. 1, one proposed method for reducing harmonics is demonstrated in U.S. Pat. No. 5,122,705. The stator winding has one set of Y-connected three-phase windings and one set of delta-connected three-phase windings. The currents in the two three-phase windings have a 30xc2x0 phase shift. The harmonic flux is generated by the windings cancel each other to some extent to reduce magnetic noise. However, the winding configuration allows fundamental current component circulating in the three loops formed by the six windings; U, X, Z; V, X, Y; and, W, Y, Z. Because the circulating current does not contribute to the output of the alternator, the efficiency of the alternator is reduced. To minimize circulating current, the ""705 patent has two significant drawbacks. First, the number of stator slots is doubled compared to existing alternators. Secondly, the selection of number of turns is limited.
To prevent circulating current from occurring, the line-to-line voltages of the Y-connected windings have to be identical to the phase voltages of the Delta-connected windings. That is, the line-to-line voltage Vxy of the Y-connected windings has to have the same magnitude and phase angle as those of the phase voltage Vv of the Delta-connected windings. Since the line-to-line voltage Vxy has a 30xc2x0 phase shift with respect to the phase voltage Vx, the voltage Vv, has to have a 30xc2x0 phase shift with respect to Vx in order for Vv to have the same phase angle with that of Vxy. In order for Vv to have a 30xc2x0 phase shift with respect to the Vx the winding V must be placed in the stator with a 30xc2x0 electrical angle displacement in space with respect to the winding X. However, the minimum displacement between two coils in existing alternators is 60xc2x0, which is the angle between two adjacent stator slots. Therefore, the number of stator slots has to be doubled in order for the wye-connected windings to have a 30xc2x0 displacement with regard to the delta-connected windings. The high number of stator slots makes the teeth very narrow and subject to deformation during the winding manufacturing or the stator core assembly. Also, the high number of stator slots makes the winding manufacturing complicated and therefore increases the alternator cost.
With respect to the second drawback related to the number of turns, in order for Vv to have the same magnitude as that of Vxy, which equals 3Vx, the number of turns of winding V(Nv) should be 3 times the number of turns of the winding X (Nx). Since the number of turns must be an integer, it is impossible to exactly satisfy the condition Nv=3 Nx, which is a necessary condition to eliminate the circulating current. Therefore, the configuration proposed in the ""705 patent cannot be made to eliminate circulating current.
U.S. Pat. No. 5,449,962 illustrated in FIG. 2 has a different winding configuration from that shown in FIG. 1. This embodiment is believed to have the second drawback described above. That is, there is little flexibility in the selection of the number of turns of the stator winding.
Therefore, it would be desirable to provide an alternator stator configuration that reduces magnetic noise and thus, audible noise without increasing the number of stator slots and without having new constraints on the selection of the number of turns.
The present invention provides a reduced noise alternator that reduces magnetic harmonic fluxes in the alternator and thus the magnetic noise of the alternator is reduced. In one aspect of the invention, an electrical machine has a stator core with a first and second and third plurality of slots. A first set of three-phase windings has a first winding, second winding, and a third winding. A second set of windings also has a first winding, a second winding and a third winding. A third set of windings also has a first winding, a second winding and a third winding. The first winding of the first set of three-phase windings and the first winding of the second set of three-phase windings and the first winding of the third set of three-phase windings are disposed in the first plurality of slots. The second winding of the first set of three-phase windings and the second winding of the second set of three-phase windings and the second winding of the third set of three-phase windings are disposed in the second plurality of slots. The third winding of the first set of three-phase windings and the third winding of the second set of three-phase windings and the third winding of the third set of three-phase windings are disposed in the third plurality of slots. The first, second, and third set of windings are grouped into a first wye, a second wye and a third wye, said first wye electrically coupled to said second wye and said third wye, said second wye electrically coupled to said third wye.
In another aspect of the invention, three sets of three-phase windings are coupled together. Each of the groupings has three separate and distinct windings. A first grouping is formed from the three sets of three-phase windings in an wye-formation. A second grouping is formed in a second wye-formation and third grouping is formed in a third wye-formation. Each of the groupings having a respective first common node, second common node and third common node there-between. One winding of the first grouping connected to one winding of the second grouping and a second winding of the first grouping connected to one winding of the third grouping. One winding of the second grouping is connected to one winding of the first grouping and one winding of the third grouping. One winding of the third grouping is connected to one winding of the first grouping and one winding of the second grouping.
One advantage of the invention is that a conventional stator core having 36 slots with 12 poles that are commonly used in current alternators may be employed. The stator cores have known characteristics and are unlikely to deform because more slots are not required.